A quasi$-$isotropic graphite$/$epoxy laminate has a fracture toughness

$K_{IC}=30$MPa$-m^{\frac{1}{2}}$ and a tensile strength of $500$ MPa $.$ As shown in figure $1,$ a

$25$ mm wide structural element made from this material has an edge crack of

length $a=3mm.$ If the element is subjected to a uniaxial stress, $,$ determine

the critical value of the stress that would cause unstable propagation of the

crack. Compare this stress with the tensile strength of the material, which does

not take cracks into account.

Figure $1$